This invention relates to semiconductor devices having as the conductive channel electrons accumulated in the neighborhood of a heterojunction interleaved between a pair of layers of semiconductors having different electron affinities, wherein the accumulated electrons have a large electron mobility. (Such a device is hereinafter referred to as a high electron mobility heterojunction semiconductor device.) More specifically, the invention enables easy adjustment, revision or modulation of the characteristics of the semiconductor devices, after production or after the semiconductor devices are put into practical use, for the ultimate purpose of providing the semiconductor devices with flexibility and to diversify their uses, particularly into the field of memory elements, and the invention further provides methods for employing such the memory elements. The invention can also increase the operating speed of high electron mobility heterojunction semiconductor devices.
A high electron mobility heterojunction semiconductor device is defined as either an active or a passive semiconductor device the conductive channel of which does not include carriers contained in one or more bulk semiconductors, but instead consists of electrons (hereinafter referred to as the two-dimensional electron gas) accumulated in a two-dimensional surface contiguous with a heterojunction that is interleaved between a layer of a first semiconductor such as AlGaAs and a layer of another semiconductor having larger electron affinity then the semiconductor of the first layer such as GaAs. Since the layer of the first semiconductor having the smaller electron affinity supplies electrons to the two-dimensional electron gas, this layer is hereinafter referred to as the electron source layer. Since the semiconductor layer having the larger electron affinity allows electrons to accumulate therein for the ultimate purpose of providing a channel for the two-dimensional electron gas, the latter layer is hereinafter referred to as the channel layer.
The outstanding feature of electrons accumulated in a two-dimensional surface contiguous with a heterojunction interleaved between an electron source layer of a first semiconductor such as AlGaAs and a channel layer of a semiconductor having larger electron affinity than the first semiconductor of the source layer such as GaAs, is that extremely high electron mobility becomes available at cryogenic temperatures, that is, within a temperature range not exceeding 150.degree. K. The thickness of the two-dimensional electron gas which accumulates in the channel layer of e.g. GaAs, in the form of a plane contiguous with the heterojunction, is extremely small, specifically less than 100.ANG., and can thus be considered for the purposes of the present disclosure to be effectively two-dimensional. Therefore, the two-dimensional electron gas has a geometrical position that is separated from the electron source layer of AlGaAs, for example. As a result, the mobility of the electrons constituting the two-dimensional electron gas becomes free from the effects of ionized-impurity scattering caused by the impurities contained in layers contiguous therewith. It is well-known that ionized-impurity scattering is the major parameter which restricts electron mobility in the cryogenic temperature range. Since the electrons constituting the two-dimensional electron gas are free from the effects of ionized-impurity scattering, as described earlier, the two-dimensional electron gas turns out to have an extremely large electron mobility at temperatures in the cryogenic temperature range. Experiments show that the magnitude of improvement in electron mobility is more than a factor of 10. Electron mobilities of 1.5.times.10.sup.5 cm.sup.2 /VS and 5.times.10.sup.5 cm.sup.2 /VS were measured respectively at 77.degree. K. and 5.degree. K. for one device having an undoped GaAs channel layer and an n-type AlGaAs electron source layer.
The requirements for pairs of semiconductors which allow the two-dimensional electron gas to accumulate therebetween are (1) an identity or resemblance in lattice constant and (2) a large difference in electron affinity and band gap. Therefore, a number of pairs are available including some examples tabulated below.
______________________________________ Semiconductor Pair Allowing the Two-dimensional Electron Gas to Accumulate Therebetween Lattice Constant Electron Affinity Item Semiconductors angstrom eV ______________________________________ 1. AlGaAs 5.657 3.77 GaAs 5.654 4.07 2. AlGaAs 5.657 3.77 Ge 5.658 4.13 3. GaAs 5.654 4.07 Ge 5.658 4.13 4. CdTe 6.477 4.28 InSb 6.479 4.59 5. GaSb 6.095 4.06 InAs 6.058 4.9 ______________________________________
A major example of active high electron mobility heterojunction semiconductor devices is a high electron mobility transistor (hereinafter referred to as a HEMT). Included in the category of passive high electron mobility heterojunction semiconductor devices are capacitors and connection channels, which are usually produced to be associated with one or more HEMTs produced in a substrate on which such capacitors and/or such connection channels are produced.
One of the drawbacks inherently associated with known high electron mobility heterojunction semiconductor devices is that the characteristics thereof cannot be adjusted, revised, modified or modulated, because the characteristics are determined by the layer configuration of the device and the concentration of n-type impurities contained in the electron source layer of the device, both of which cannot be adjusted, revised, modified or modulated, once the device is produced. If the characteristics of a high electron mobility heterojunction semicoductor device could be adjusted, revised, modified or modulated, this would be effective (1) to improve the productivity, and specifically the production yield, of the device, (2) to provide a high electron mobility heterojunction semiconductor device which is capable of adjustment, revision or modulation of the characteristics thereof, and (3) to diversify the uses thereof particularly to the field of a memory element, and specifically of a programmable memory element, a switching element, a sensing element, etc.
The other drawbacks inherently associated with high electron mobility heterojunction semiconductor devices is that the electron mobility of the two-dimensional electron gas is more or less influenced by the ionized impurity scattering phenomenon caused by the ionized impurities existing in the neighborhood of the heterojunction. On the other hand, some type of source of electrons which are free or which are not held by atoms is essential for the electron source layer, because the two-dimensional electron gas requires some quantity of electrons from one of the semiconductor layers constituting a heterojunction along which the two-dimensional electron gas accumulates. In order to satisfy this requirement, the high electron mobility heterojunction semiconductor devices available in the prior art are provided with an electron source layer of a semiconductor having smaller electron affinity than the semiconductor of the channel layer and which contains an n-type impurity. Therefore, one of the potentially easy methods to further increase the electron mobility of the two-dimensional electron gas may be to increase the surface concentration of electrons which constitute the two-dimensional electron gas, because this is effective to increase the screening of the scattering potential, resulting in decreasing the effects of ionized impurity scattering by the impurities contained in the semiconductor layers constituting the heterojunction along which the two-dimensional electron gas accumulates. Increase in the impurity concentration of the semiconductor of an electron source layer is effective to increase the surface concentration of electrons which constitute the two-dimensional electron gas. However, this is inevitably accompanied by an adverse effect in which some quantity of the impurities contained in the electron source layer is diffused into the channel layer during thermal processes which may be carried out in one or more later steps, resulting in increasing the effects of ionized impurity scattering caused by ionized impurities contained in the channel layer, and ultimately offsetting the effects of the increased surface concentration of electrons which constitute the two-dimensional electron gas.
In order to remove this drawback, some of the high electron mobility heterojunction semiconductor devices are provided with a buffer layer containing no impurities and which is produced of a semiconductor which is identical to that of the electron source layer. The thicker is the buffer layer, the greater is the effect of prohibiting the impurity diffusion to the channel layer. On the other hand, a thicker buffer layer reduces the effects of the potential gap caused by the difference in electron affinity, resulting in decreasing the surface concentration of the two-dimensional electron gas. This means that, if it is possible to decrease the concentration of n-type impurities contained in the electron source layer without being accompanied by an adverse effect of decreasing the surface concentration of electrons which constitute the two-dimensional electron gas, this would be effective to increase the electron mobility of the two-dimensional electron gas.